The invention relates to an apparatus for printing a flexible web, in particular a paper web, comprising a positively driven printing base, printing cylinders capable of pressing against a web moving between the printing cylinders and the printing base, and a drive for removing the printed web.
In devices for multicolor printing of webs it is of greatest importance during the printing operation to avoid any relative displacement between the web and the printing base which runs synchronously with the printing rollers. Otherwise, the separately applied inks would be shifted in relation to one another. In textile printing, the achievement of full synchronization of web and printing is provided in that the web is glued to the printing base during the printing operation. In the case of paper, however, the same procedure would lead to the destruction of the material. Accordingly, during the printing and winding up of paper web material, attempts have been made to exclude as mush as possible the appearance of forces acting in the direction of a displacement of the web in relation to the printing base. In devices of the type that are not based on the principle of magnetic pressure application, this has been achieved by leading the web around a presser device having the printing rollers distributed over its circular periphery. The friction between the web and the presser, which is caused by the tension of the web, prevents an excessively easy slipping of the web. Yet, the drive of the web and the drive of the presser must be synchronized very carefully, since the friction forces obtainable through the tension of the web are small, and the printing rollers acting with a low pressure on the web do not substantially contribute to a real friction connection between the web and the presser.
In another device of the type described, a rotating blanket in the manner of a conveyor belt is associated with a plurality of printing cylinders. The blanket runs in a plane (i.e. has a flat run) between the printing cylinders that ordinarily consist of screen printing stencils having magnetic rollers arranged in the interior thereof. Tension produced in the web running over the flat area of the blanket would not at all lead to the appearance of friction forces between the blanket and the web in this case. On the basis of the consideration that variations in the tension of the web would thus necessarily have to lead to displacement between the web and the blanket, until the present time there has been placed great importance on avoiding any tension in the web in the area of the blanket. Consequently, the unreeling or supply spool of the web has been provided with a drive, i.e. the web was pushed to a certain extent into the printer. Behind or downstream of the printing rollers, the web was allowed initially to sag untensioned, in which connection the degree of sagging was employed for controlling a drive for the winding-up spool. Owing to the difficulties in synchronizing the three drives that are provided in such devices, it is not possible to eliminate the danger of accumulation of substantially untensioned web, so that the theoretically obtainable printing speed of the printing system cannot be maintained over relatively long periods of time.